1. Field of the Invention
The invention relates to an alkali-free aluminoborosilicate glass. The invention also relates to uses of this glass.
2. Background of the Invention
High requirements are made of glasses for applications as substrates in flat-panel liquid-crystal (or expressed differently: liquid crystal) display technology, for example in TN (twisted nematic)/STN (supertwisted nematic, or expressed differently: super twisted nematic) displays, active matrix liquid crystal displays (AMLCDs), thin-film transistors (TFTs) or plasma addressed liquid crystals (PALCs). Besides high thermal shock resistance and good resistance to the aggressive chemicals employed in the process for the production of flat-panel screens, the glasses should have high transparency over a broad spectral range (VIS, UV) and, in order to save weight, a low density. Use as substrate material for integrated semiconductor circuits, for example in TFT displays (xe2x80x9cchip on glassxe2x80x9d) in addition requires thermal matching to the thin-film material silicon which is usually deposited on the glass substrate in the form of amorphous silicon (a-Si) at low temperatures of up to 300xc2x0 C. The amorphous silicon is partially recrystallized by subsequent heat treatment at temperatures of about 600xc2x0 C. Owing to the a-Si fractions, the resulting, partially crystalline poly-Si layer is characterized by a thermal expansion coefficient of xcex120/300xe2x89xa13.7xc3x9710xe2x88x926/K. Depending on the a-Si/poly-Si ratio, the thermal expansion coefficient xcex120/300 may vary between 2.9xc3x9710xe2x88x926/K and 4.2xc3x9710xe2x88x926/K. When substantially crystalline Si layers are generated by high temperature treatments above 700xc2x0 C. or direct deposition by CVD processes, which is likewise desired in thin-film photovoltaics, a substrate is required which has a significantly reduced thermal expansion of 3.2xc3x9710xe2x88x926/K or less.
In addition, applications in display and photovoltaics technology require the absence of alkali metal ions. Sodium oxide levels of less than 1000 ppm (parts per million) as a result of production can be tolerated in view of the generally xe2x80x9cpoisoningxe2x80x9d action due to diffusion of Na+ into the semiconductor layer.
It should be possible to produce suitable glasses economically on a large industrial scale in adequate quality (no bubbles, knots, inclusions), for example in a float plant or by drawing methods. In particular, the production of thin ( less than 1 mm) streak-free substrates with low surface undulation by drawing methods requires high devitrification stability of the glasses. Compaction of the substrate during production, in particular in the case of TFT displays, which has a disadvantageous effect on the semiconductor microstructure, can be countered by establishing a suitable temperature-dependent viscosity characteristic line of the glass: with respect to thermal process and shape stability, it should have a sufficiently high glass transition temperature, i.e. Tg greater than 700xc2x0 C., while on the other hand not having excessively high melting and processing (VA) temperature, i.e. a VA of xe2x89xa61350xc2x0 C.
The requirements of glass substrates for LCD display technology or thin-film photovoltaics technology are also described in xe2x80x9cGlass substrates for AMLCD applications: properties and implicationsxe2x80x9d by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997), and in xe2x80x9cPhotovoltaikxe2x80x94Strom aus der Sonnexe2x80x9d by J. Schmid, Verlag C. F. Mxc3xcller, Heidelberg 1994, respectively.
The abovementioned requirement profile is fulfilled best by alkaline earth metal aluminoborosilicate glasses. However, the known display or solar cell substrate glasses described in the following publications still have disadvantages and do not meet the full list of requirements:
Numerous documents describe glasses having low MgO and/or CaO contents: Japanese Patent Application No. 9-169 538 A, Japanese Patent Application No. 4-160 030 A, Japanese Patent Application No. 9-100 135 A, European Patent Application No. 714 862 A1, European Patent Application No. 341 313 B1, U.S. Pat. No. 5,374,595, Japanese Patent Application No. 9-48632 A, Japanese Patent Application No. 8-295530 A, International Application No. 97/11919 and International Application No. 97/11920. These glasses, do not have the desired meltability, as is evident from very high temperatures at viscosities of 102 dPas and 104 dPas, and have a relatively high density. The same applies to the MgO-free glasses of DE 37 30 410 A1, U.S. Pat. No. 5,116,787 and U.S. Pat. No. 5,116,789.
On the other hand, glasses having high MgO contents, as described in Japanese Patent Application No. 61-123 536 A, are insufficient in terms of their chemical resistance and their devitrification and segregation behavior.
The glasses described in International Application No. 98/27019 contain very little BaO and SrO and are likewise susceptible to crystallization.
Glasses having a high content of the heavy alkaline earth metals BaO and/or SrO, as described in European Patent Application No. 341313 B1, have undesirably high densities and poor meltabilities. The same is true for the glasses of Japanese Patent Application No. 10-72237 A. According to the examples, the glasses have high temperatures at viscosities of 104 dPas and 102 dPas.
Glasses having low boric acid contents likewise exhibit excessively high melting temperatures or, as a result of this, excessively high viscosities at the melt and processing temperatures required for processes involving these glasses. This applies to the glasses of Japanese Patent Application No. 10-45422 A, Japanese Patent Application No. 9-263421 A and Japanese Patent Application No. 61-132536 A.
Moreover, glasses of this type have a high devitrification tendency when combined with low BaO contents.
In contrast, glasses having high boric acid contents, as described, for example, in U.S. Pat. No. 4,824,808, have insufficient heat resistance and chemical resistance, in particular to hydrochloric acid solutions.
Glasses having a relatively low SiO2 content do not have sufficiently high chemical resistance either, in particular when they contain relatively large amounts of B2O3 and/or MgO and are low in alkaline earth metals. This applies to the glasses of International Application No. 97/11919 and European Patent Application No. 672 629 A2. The relatively SiO2-rich variants of the latter document have only low Al2O3 levels, which is disadvantageous for the crystallization behavior.
The glasses described in Japanese Patent Application No. 9-12333 A for hard disks, are comparatively low in Al2O3 or B2O3, the latter merely being optional. The glasses have high alkaline earth metal oxide contents and have high thermal expansion, which makes them unsuitable for use in LCD or PV technology.
DE 42 13 579 A1 describes glasses for TFT applications having a coefficient of thermal expansion xcex120/300 of  less than 5.5xc3x9710xe2x88x926/K, according to the examples of xe2x89xa74.0xc3x9710xe2x88x926/K. These glasses which have relatively high B2O3 levels and relatively low SiO2 contents do not have a high chemical resistance, in particular to diluted hydrochloric acid.
DE 196 01 022 A1 describes glasses which are selected from a very wide composition range and which must contain ZrO2 and SnO. These low-Al2O3 glasses tend to exhibit glass defects because of their ZrO2 level.
Federal Republic of Germany Patent No. 196 17 344 C1 (U.S. Pat. No. 5,908,703) and Federal Republic of Germany Patent No. 196 03 689 C1 (U.S. Pat. No. 5,770,535) by the Applicant disclose alkali-free, tin oxide-containing, low-SiO2 or low-Al2O3 glasses having a coefficient of thermal expansion xcex120/300 of about 3.7xc2x710xe2x88x926/K and very good chemical resistance. They are suitable for use in display technology. However, since they must contain ZnO, they are not ideal, in particular for processing in a float plant. In particular at higher ZnO contents ( greater than 1.5% by weight), there is a risk of formation of ZnO coatings on the glass surface by evaporation and subsequent condensation in the hot-shaping range.
JP 9-156 953 A also relates to alkali-free glasses for display technology which are low in Al2O3. The heat resistance of these glasses is insufficient, as is evident from the glass transition temperatures of the exemplary glasses.
In the unexamined Japanese publications Japanese Patent Application No. 10-25132 A, Japanese Patent Application No. 10-114538 A, Japanese Patent Application No. 10-130034 A, Japanese Patent Application No. 10-59741 A, Japanese Patent Application No. 10-324526 A, Japanese Patent Application No. 11-43350 A, Japanese Patent Application No. 10-139467 A, Japanese Patent Application No. 10-231139 A and Japanese Patent Application No. 11-49520 A, mention is made of very wide composition ranges for display glasses, which can be varied by means of many optional components and which are admixed with one or more specific refining agents in each case. However, these documents do not indicate how glasses having the complete requirement profile described above can be obtained in a specific manner.
It is an object of the present invention to provide glasses which meet said physical and chemical requirements imposed on glass substrates for liquid-crystal displays, in particular for TFT displays, and for thin-film solar cells, in particular on the basis of xcexcc-Si, glasses which have high heat resistance, a favorable processing range and sufficient devitrification stability.
The invention teaches that this object can be achieved by aluminoborosilicate glasses with the following composition (in % by weight, based on oxide): silicon dioxide (SiO2)xe2x80x94from somewhat greater than 58% to 65% ( greater than 58%-65%); boric oxide (B2O3)xe2x80x94from somewhat greater than 6% to 11.5% ( greater than 6%-11.5%); aluminum oxide (Al2O3)xe2x80x94from somewhat greater than 20% to 25% ( greater than 20%-25%); magnesium oxide (MgO)xe2x80x94from 4% to somewhat less than 6.5% (4% - less than 6.5%); calcium oxide (CaO)xe2x80x94from somewhat greater than 4.5% to 8% ( greater than 4.5%-8%); strontium oxide (SrO)xe2x80x94from 0% to somewhat less than 4% (0%- less than 4%); barium oxide (BaO) from 0.5% to somewhat less than 5% (0.5%- less than 5%); with strontium oxide (SrO)+barium oxide (BaO)xe2x80x94to more than 3% ( greater than 3%); and zinc oxide (ZnO)xe2x80x94from 0% to 2%(0 %-2%).
The glass contains between  greater than 58 and 65% by weight of SiO2. At lower contents, the chemical resistance is impaired, while at higher levels, the thermal expansion is too low and the crystallization tendency of the glass increases. Preference is given to a maximum content of 64.5% by weight.
The glass contains from  greater than 20 to 25% by weight of Al2O3. Al2O3 has a positive effect on the heat resistance of the glass without excessively increasing the processing temperature. At a low content, the glass becomes more susceptible to crystallization. Preference is given to a content of at least 20.5% by weight, in particular of at least 21% by weight, of Al2O3. Preference is given to a maximum Al2O3 content of 24% by weight.
The B2O3 content is restricted to a maximum of 11.5% by weight in order to achieve a high glass transition temperature Tg. Higher contents would also impair the chemical resistance. Preference is given to a maximum B2O3 content of 11% by weight. The B2O3 content is higher than 6% by weight to ensure that the glass has good meltability and good crystallization stability.
An essential glass component are the network-modifying alkaline earth metal oxides. In particular by varying their levels, a coefficient of thermal expansion xcex120/300 of between 2.8xc3x9710xe2x88x926/K and 3.6xc3x9710xe2x88x926/K is achieved. The individual oxides are present in the following proportions:
The glass contains from 4 to  less than 6.5% by weight of MgO and from  greater than 4.5 to 8% by weight of CaO. Rather high levels of these two components have a positive effect on the desired properties of low density and low processing temperature, whereas rather low levels favor crystallization stability and chemical resistance.
The glass furthermore contains BaO, specifically at least 0.5% by weight. The maximum BaO content is limited to less than 5.0% by weight. This ensures good meltability and keeps the density low.
The glass may furthermore contain up to  less than 4% by weight of the relatively heavy alkaline earth metal oxide SrO. Limitation of these optional components to this low maximum content is especially advantageous for a low density and good meltability of the glass. In order to improve the crystallization stability, it is preferred that SrO is present, specifically preferably in an amount of at least 0.2% by weight.
The total content of BaO and SrO is in this case  greater than 3% by weight in order to ensure sufficient crystallization stability.
The glass may contain up to 2% by weight of ZnO, preferably  less than 2% by weight of ZnO. The network modifier ZnO has a structure-loosening function and has less effect on the thermal expansion than the alkaline earth metal oxides. Its effect on the viscosity characteristic line is similar to that of B2O3. In particular in the case of processing of the glass by the float process, the ZnO level is preferably limited to a maximum of 1.5% by weight. Higher levels would increase the risk of unwanted ZnO coatings on a glass surface which may form by evaporation and subsequent condensation in the hot-shaping range.
The glass is alkali-free. The term xe2x80x9calkali-freexe2x80x9d as used herein means that it is essentially free from alkali metal oxides, although it can contain impurities of less than 1000 ppm (parts per million).
The glasses may contain up to 2% by weight of ZrO2+TiO2, where both the TiO2 content and the ZrO2 content can each be up to 2% by weight. ZrO2 advantageously increases the heat resistance of the glass. Owing to its low solubility, ZrO2 does, however, increase the risk of ZrO2-containing melt relicts, so-called zirconium nests, in the glass. ZrO2 is therefore preferably omitted. Low ZrO2 contents originating from corrosion of zirconium-containing trough material are unproblematic. TiO2 advantageously reduces the solarization tendency, i.e. the reduction in transmission in the visible wavelength region because of UV-VIS radiation. At contents of greater than 2% by weight, color casts can occur due to complex formation with Fe3+ ions which are present in the glass at low levels as a result of impurities of the raw materials employed.
The glasses may contain conventional refining agents in the usual amounts: they may thus contain up to 1.5% by weight of As2O3, Sb2O3, SnO2 and/or CeO2. It is likewise possible to add 1.5% by weight each of Clxe2x88x92 (for example in the form of BaCl2), Fxe2x88x92 (for example in the form of CaF2) or SO42xe2x88x92 (for example in the form of BaSO4). The sum of As2O3, Sb2O3, CeO2, SnO2, Clxe2x88x92, Fxe2x88x92 and SO42xe2x88x92 should, however, not exceed 1.5% by weight.
If the refining agents As2O3 and Sb2O3 are omitted, the glasses can be processed not only using the various drawing methods, but also by the float method.
For example with regard to easy batch preparation, it is advantageous to be able to omit both ZrO2 and SnO2 and still obtain glasses having the property profile mentioned above, in particular having high heat and chemical resistance and low crystallization tendency.
The above-discussed embodiments of the present invention will be described further hereinbelow. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is, the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintains that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.